Driver circuits often include a feedback loop to increase noise immunity and to improve output waveforms by means of hysteresis. Typically, a driver circuit includes a pair of inverting buffers connected in series, and hysteresis is introduced by means of a third inverting buffer which couples the output of the second inverting buffer back to an inter-buffer node at the input of the second inverting buffer. Positive feedback to this inter-buffer node shifts the threshold voltage at which the driver circuit output switches between states and speeds the transition between states once the switching point has been reached.
A number of undesirable side effects result when hysteresis is introduced into a circuit in the manner described above. For example, the threshold voltage of the driver circuit and the level of hysteresis are sensitive to variations in the supply voltage. In addition, signal propagation through the driver circuit is delayed since the driving signal must overcome the feedback signal at the inter-buffer node before the second buffer can begin to switch states.